The present invention relates to a plant for the hot treatment of waste gases from municipal waste incinerators or the like.
At this time, as is known, there is an ever growing frequency of combined applications, whereby hot waste gases originating from municipal waste incinerators are treated to reduce their polluting emissions and simultaneously used, thanks to their high thermal energy content, for instance in steam plants for generating electrical power. As is also known, said steam plants often employ one or more super-heaters downstream of the boiler producing saturated steam, for the purpose of obtaining superheated steam to be subsequently sent to expand in a steam turbine, coupled with an electric power generator.
A most important problem in this type of applications is represented by the fact that in the municipal waste incinerators treating the waste gases at high temperatures, above 850xc2x0 C., downstream of the incinerator afterburner, are characterized by very high concentrations of acids and oxides, for instance hydrochloric acid present in volumes of the order of 0.3-3 g/Nm3 and sulfur oxides present in volumes of the order of several hundreds of mg/Nm3. These presences of acids and oxides in the waste gases prevent raising the superheated steam temperature over 350-380xc2x0 C., so at avoid incurring in super-heater corrosion phenomena primed by higher temperatures, which lead to said super-heaters"" failure in the short term, from a few weeks to a few months.
In my previous Italian Patent No. 1,299,828 filed Feb. 3, 1998 and granted Apr. 4, 2000, a circulating fluid bed hot treating reactor of waste gases for the recovery of energy is described. Said reactor is schematically shown in FIG. 1 of the attached drawings, wherein the numeral 101 designate an incinerator afterburner emitting two hot waste gas streams: a first stream is conveyed to the chimney 122 by an aspirator 121 after passing a filtration system 119, while a second stream is conveyed through an aspirating line 102 to a reactor 106. As can be observed, said second stream of waste gases is split into four gas entrance sectors to the reactor 106. In these sectors the waste gases are subjected to a first pre-treatment to knock out certain polluting substances through a dosing device injecting the feeding lines 105 with reagents based on ammonia, urea and/or other materials capable of neutralizing the nitrogen oxides present in the sour waste gases. The waste gases thus pre-treated are then fed to the fluid bed reactor 106 at various levels: the first injection occurs at the lowest level, opposite the chamber 107, the second injection occurs in the region 109 wherein the bed is prevalently fixed, the third injection occurs in the region 111 wherein the fluid bed is prevalently boiling, and finally the fourth injection occurs in the region 112, wherein the fluid bed is prevalently circulating. The fixed bed region 109 is formed by an inert solid granular material resting on a fluidising grid 108. By summing up, the hot gas stream originating from the chamber 107 fluidises, by passing through the grid 108, the bed of granular material of the fixed bed region 109, wherein the first stage super-heater 110 is immersed. At the level of the second injection region 111, where the fluid bed is prevalently boiling, the gaps between the granular material""s solid particles are wider, while in the third circulating bed region 112 said gaps are even wider. In the first fixed bed region 109, a heat exchange occurs between the hot waste gases and the steam flowing in the super-heater 110: the granular material contained inside said region 109 serves the function of an intermediate heat carrier between said waste gases and the walls of said super-heater 110, thus preventing the already pre-treated waste gases from coming into intimate contact with the walls of the super-heater 110. This achieves the dual result of heating up the steam flowing in the super-heater, while at least partially preventing the waste gases from directly contacting the super-heater walls 110, and from lowering the temperature of the waste gases which are going to vertically cross the reactor 106 toward the regions 111 and 112, wherein the stages of the super-heater 110 are ever less xe2x80x9cprotectedxe2x80x9d by the granular material, because of the fact that, as mentioned, the gaps between the particles of this material are increasing. At the outlet of the reactor 106, the waste gases, which also entrain the fluid bed""s lightest particles, are conveyed by the line 113 to a cyclone separator 114, that recirculates the coarsest particles through the line 115 to the reactor 106, while the finest fraction of the same, considered to be exhausted, is removed by an aspirating device 117, that conveys said exhausted particles to the filtration device 119 and hence to the chimney 122.
The above mentioned reactor, assumed to be the state of the art, evidenced the lack of a recirculating system capable of allowing a further exploitation of the waste gases exiting the cyclone separator 114, as they are generally carrying a considerable residual energy content; the presence of complex injection ramifications of the waste gases inside the reactor 106, the high thermal stresses and jumps to which the only super-heater 110 included in the three-stage fluid bed reactor is subjected; and the usage of a waste gas pre-treatment by introducing regents such as ammonia and/or urea into the feedlines to the reactor.
The purpose of the present invention is to realize a plant for the hot treatment of waste gases from municipal waste incinerators or the like, which would, starting from a reactor of a type outlined in my previous Italian Patent No. 1,299,828, be capable of: obtaining superheated steam above 400xc2x0 C., by increasing the efficiency of the electrical energy producing cycle associated with the same, with evident advantages from an economical viewpoint; avoiding the immission into the atmosphere of heavily polluting emissions on the part of the hot waste gases, by effectively knocking out the noxious and corrosive substances contained in said waste gases, with evident advantages from an environmental viewpoint; and moreover, preventing said corrosive substances from reaching the super-heater walls while compromising their duration and operating efficiency.
The above object is achieved by the present invention, through a combination plant for the hot treatment of the waste gases from municipal waste incinerators or the like, comprising a first uptake line of the hot waste gases to be treated from an exit section of a municipal waste incinerator, a single or a multiple stage fluid bed contained within one or more reactors and invested by the waste gases to be treated, at least one hopper for charging the solid granular material to the reactor or reactors, a cyclone separator at the outlet of the reactor or of the last of the reactors; the treated waste gases exiting from the reactor or reactors being first introduced into said separator and subsequently conveyed, by said first uptake line, to a final discharging chimney; wherein said first uptake line, downstream of said cyclone separator, includes a second recirculating line that conveys the waste gases treated by the reactor or reactors, upstream of the reactor or of the first of the reactors, so that by this second line these treated waste gases mix with the waste gases to be treated originating from the incinerator""s uptake section.
Moreover, at least one super-heater is placed within the reactor or reactors, which is passed by the steam to be heated and sent to an electrical energy producing cycle.